The objective of this study is a detailed characterization of the cardiovascular status of a unique group of patients who have previously sustained an episode of sudden and unexpected cardiac arrest in the community, and who are at high risk for subsequent potential lethal arrhythmias and sudden death. The characterization includes detailed studies of (1) the acute and chronic electrophysiologic status of the patients, (2) the hemodynamic status of the patients, (3) coronary artery anatomy, (4) determination of the presence of recent or old myocardial infarctions, and evidence for chronic myocardial ischemia, and (5) the factors which determine survival in a sub-group of long-term survivors. The patient population being studied are those individuals who were successfully resuscitated from a sudden and unexpected cardiac arrest in the community. Studies being performed include intracardiac and non-invasive electrophysiologic studies, both in the acute setting and in the long-term survivors, cardiac catheterization and coronary angiography, the cardiac response to exercise, radionuclide imaging techniques for the determination of myocardial damage, and an evaluation of the effect of interventions upon identifiable risk factors.